In the reception of quantized signals, the received signal states are detected by means of references representing predetermined reference states. These references are conventionally set to correspond to the ideal states of the quantized signal in question. A quantized signal may have these ideal states when it is generated, for example in the modulator of a transmitter. However, the transmission channel between a transmitter and a receiver typically introduces both linear and non-linear distortion on a signal, which distorts the signal states of a received signal so that it is difficult or impossible to detect them using an ideal reference set. Conventional attempts to obviate such distortion introduced by the transmission channel have included processing the received signal with a specific equalizer before it is supplied to the detector so that the signal to be detected would correspond to the original signal transmitted as accurately as possible.
The Applicant's previous patent application PCT/FI89/00037 discloses an adaptive detection method wherein the reference values used in the detection are continually corrected towards the actual received signal states, whereby the reference set can differ to a great extent from the ideal reference set. Patent application PCT/FI89/00037 gives examples of suitable methods for updating the reference values of an adaptive detector: the method employing a self-organizing map, and the K-means method. Of these, the K-means method is ideally unbiased with signals varying relatively regularly; in this method the references tend to follow the averages of actual signal states accurately. In other words, when the noise of a received signal and the variations of the signal levels are not very great, more accurate values are given for the references by the K-means method than by the map method. From the point of view of operation as a detector, the K-means method has however a disadvantageous property: when the signal levels of a received signal change rapidly and a certain signal state does not occur for a long time, the references may start to follow incorrect signal states. The K-means method cannot "recognize" this error and does not recover from it except by accident, most likely after a long time. Incorrect reference values naturally lead to complete failure of the detection. Under the above-mentioned conditions, where the noise of a received signal and/or variations in the signal level become great or sudden, the method based on a self-organizing map is clearly more suitable, as it can prevent the reference values from "escaping" and being assigned with incorrect signal states, and even rapidly restore the correct reference values. Patent application PCT/FI89/00037 suggests that under difficult conditions a solution be used where the map method is employed at the beginning of the reception for roughly finding the reference values, and where the K-means method is employed for the rest of the reception time in order to make the reference values follow up the received signal states as accurately as possible. If the detection fails later for some reason, the correct reference values can be restored again by using the map method temporarily. This known solution is thus a compromise where the (desirable and undesirable) properties of two different methods alternate with time; they are however not effective simultaneously at any span of time.